Recently, besides the gloss intensity of surfaces of products, the unevenness of gloss and that of printed coated paper or films have been regarded important as the quality of the products The qualities are, for example, "surface feeling" of coated paper, "flowmarks" of the surface of formed plastics, and "clear reflective property" of automobile applications
About the surface feeling of coated paper, it is considered that the more uniform the gloss value of the product appearance, the better the surface feeling. The evaluation has been conventionally made by inspectors who conducts visual observation mainly of the sub-millimeter gloss irregularity on the coated paper. Alternatively, the following test methods have been proposed, in which images of product appearance are picked up with an image pickup device like a TV camera, and the luminance level distribution (the reflected light power) of the product appearance is calculated on the basis of the acquired images.
One of the methods remodeled a chromato-scanner to scan the surface of coated paper with a light flux of 0.4 mm.phi. at angles of incidence and reflection of 75 degrees, and has disclosed that the standard deviation of reflected light power has a relation to the rank order of visual inspection (H. Fujiwara et al., 1990, TAPPI Coat. Conf. Proc., 209 (1990)).
Another method is to obtain the correlation between the average of visually observed rank order and the result of the analysis of a two-dimensional image formed by surface reflection from printed paper, which was acquired by a CCD camera. This method has disclosed that the rank order exhibits good correlation with the reflected light power, that is, with the variation coefficient of gradation. However, it has been reported that there were many cases which exhibited no correlation about the gloss irregularity of white paper (M. A. MacGregor et al., 1991, TAPPI Coat. Conf. Proc., 495 (1991)).
Next, the printing unevenness of printed coated-paper will be explained. The printing unevenness is considered to increase with the ink density variation in neighborhoods in an area to be printed uniformly. The degree of unevenness is considered to correspond to the magnitude of difference among them. Since the printing unevenness is likely to appear in halftone regions of uniform ink density, the visual evaluation is carried out by an inspector on the halftone areas of the printed matter. Several measuring methods and devices have also been proposed to detect the printing unevenness.
One of the methods is to calculate areal ratios of individual dots in the halftone areas, and derive the coefficients of variation (Takao Arai, Magazine of the Paper and Pulp Engineering Institute, 43 575 (1989)),
Another method is to conduct the line scanning of the series of dotted regions in a halftone portion by an image analyzer to digitize the regularity of density profile, thereby obtaining parameters of the printing unevenness (T. Koskinen et al., Paperi JaPuu 74 45 (1992)).
The visual inspections of the gloss irregularity and printing unevenness, however, lack quantitative accuracy because they depend on personal subjectivity, and hence it requires many professional inspectors to carry out objective judge on the quality of printing.
On the other hand, the above mentioned optical test methods of gloss irregularity have the following drawbacks.
Although the quantification of optical measurement of the gloss irregularity is tried using reflected light power distribution, it has a problem that the results do not necessarily coincide with those of the visual inspection. In particular, there are large errors in the optical method in the case where white paper surface feeling (the surface feeling before printing) is tested by measuring the power distribution of reflected light. This is because the intense diffused reflected light weakens the gloss irregularity associated with the true reflected light.
Moreover, in the optical test of the printing unevenness, there are some cases which do not coincide with the results of the visual inspection because the optical method measures ink density only.
Taking account of these facts, an object of the present invention is to provide an optical measuring method and apparatus of the gloss irregularity which can quantitatively express the measured results of the gloss irregularity with accuracy in terms of magnitude and distribution of the gloss irregularity rather than the power distribution of light, and to offer a measuring method and apparatus of the printing unevenness which can improve the accuracy by measuring the areas and distribution of uneven portions of ink density rather than by measuring the ink density itself.
Other objects and features of the present invention will be apparent from the following description, some of which will become clear by the description, or be understood by implementing the present invention. The objects and advantages of the present invention are implemented by the means recited in the claims and the combinations thereof.